There are many applications for monitoring objects or environments from remote locations. Such applications include identification and monitoring of objects such as files, records, books, equipment or other articles; identification and monitoring of biological tissue; or identification and monitoring of animals, insects or people. Other applications include remote monitoring of status information or sensor readings associated with each object to be monitored, such as biomedical information, information concerning whether an object has been moved or otherwise tampered with, or other status information concerning an object. Still other applications include remote monitoring of an environment for parameters such as movement, heat, light, sound, weather related parameters, or presence of airborne particles or chemicals.
One type of remote monitoring system includes a set of transponders that are attached or otherwise associated with the target objects or environments to be monitored. Each transponder carries information about the object or the environment with which it is associated. Upon receipt of an interrogation signal, the transponder generates and transmits a response. A base station interrogates the transponders, the transponders generate and transmit their responses, and the base station receives and interprets their response.
In some applications, objects are to be monitored in environments where a line-of-sight between the transponder and the base station may not always be available. Such environments may include open-field, forested, mountainous or aquatic outdoor environments in which obstructions such as trees, hills, waves or other variations may potentially interfere with communication. In urban and indoor environments, the obstructions may be buildings, walls, furniture or vehicles.